ORTHOGONAL BASEMENT FAULT SYSTEMS AND DIACHRONOUS OUACHITA-APPALACHIAN THRUSTING AROUND THE BLACK WARRIOR BASIN, ALABAMA AND MISSISSIPPI

The Black Warrior basin (BWb) in northern Alabama and Mississippi is bounded on the southeast by the Appalachian thrust belt and on the southwest by the Ouachita thrust belt. Both thrust belts represent late Paleozoic Alleghanian thrusting; however, in the subsurface beneath the Gulf Coastal Plain in eastern Mississippi, the Appalachian frontal faults truncate the Ouachita thrust belt, indicating diachronous thrusting.

Beneath the Appalachian allochthon, a northeast-trending system of basement faults defines a horst-and-graben system that localized the orientation of northeast-striking Appalachian frontal ramps. Transverse zones in the Appalachian thrust belt are aligned with northwest-striking basement cross faults. Thickness and facies variations show that the basement faults originated in Cambrian time as the supercontinent Rodinia broke up. The basement faults were episodically reactivated at several times during the Paleozoic, including during the Alleghanian northwestward translation of the Appalachian thrust belt.

Paleozoic strata in the BWb dip southwestward toward the Ouachita thrust front. A system of northwest-striking, mostly down-to-southwest, basement normal faults breaks the southwest-dipping homocline, displacing late Paleozoic strata. Whether these are reactivated pre-existing faults is unconstrained by available data. These basement faults generally parallel the Ouachita thrust front and are perpendicular to the Appalachian thrust belt, suggesting that they represent a brittle component of foreland subsidence in response to emplacement of the Ouachita thrust load on the crust along the southwest side of the BWb and, therefore, that they probably pre-date Appalachian thrusting.

The northwest-striking basement faults can be projected toward an intersection with the northeast-striking basement faults beneath the Appalachian frontal thrusts (and also beneath the Gulf Coastal Plain) in western Alabama, northeast of the truncation of the Ouachita thrust belt; however, the structural and temporal relationships have not been adequately documented. A set of balanced cross sections, based on well data and seismic reflection profiles, provides a three-dimensional view of the basement fault systems, as well as the relation of both systems to Appalachian structures.